In OpenOffice the spellchecker uses the first portion of the Extras menu for itself and LangaugeTool places itself as a sub-menu within that portion. I was guessing it would be the same with LibreOffice.

However, I'd swear the whole LanguageTool menu wasn't there before. Unfortunately, I cannot reproduce that again. I've just had a blue screen which reset several LibreOffice settings. And now it's there. I'll report back if I ever manage to reproduce this again.

This is quite a fresh LibreOffice installation that I manage myself. So I have no idea what could have been wrong. Trying to reproduce the issue would mean to deinstall LibreOffice, remove all LibreOffice profile data and reinstall LibreOffice again. I can do that, but not within the next days. FTR, this is LibreOffice 5.3.0.3 (x64) with LanguageTool 3.6 on Windows 7 (x64).